"Most of the patients I see with the flu or complications of the flu were not vaccinated," he said.

Once infected, Tamiflu can lessen symptoms and shorten the duration of the virus if caught within 48 hours of the onset of symptoms, Cano said.

Over-the-counter medicines can help patients feel better, but they do not prevent the development of pneumonia, he said. Patients should get plenty of fluids and rest and watch their symptoms closely.

"Shortness of breath and feeling sicker could mean the patient is developing pneumonia complications, and he might need a chest X-ray," Cano said.

Citizens Medical Center is currently treating six flu patients in-house, according to an email from Shannon Spree, marketing coordinator for the hospital.

Since Dec. 1, 2013, 189 patients have tested positive for the flu at Citizens Medical Center, Spree wrote. That number does not include flu-like cases that tested negative.

Joe Galvan Jr., 42, died Jan. 2 of sepsis, which likely developed from pneumonia. The retired Marine tested negative for the flu, as did his brother, who also developed pneumonia.

"They treated us for the flu anyway because there are so many false negatives," said his brother, Allen Galvan, 35.

The rapid diagnostic test for flu, especially the H1N1 strain, is not as reliable as some of the other options that take longer, said Christine Mann, press officer for the Texas Department of State Health Services.

DeTar Healthcare System is also seeing an increase in emergency room visits and admissions among people with flu-like symptoms.

"Influenza can be severe, and an annual vaccination is the best tool for flu prevention," wrote Judith Barefield, marketing director for DeTar Healthcare System, in an email.

The hospital advises that everyone at least 6 months old get an annual flu shot, particularly those who are at high risk of flu complications, Barefield wrote.

People at high risk for developing serious complications include children younger than 5, adults older than 65, pregnant women and individuals with medical conditions such as asthma, diabetes and chronic lung disease.